Method and Apparatus for Transmission Line Testing

ABSTRACT

A method and device for transmission line testing are provided. A particular modem device includes a transmission line interface to connect to a transmission line. The modem device also includes a memory to store a plurality of transmission line tests. The modem device also includes a controller coupled to the memory and to the transmission line interface. The controller selects one or more tests of the plurality of transmission line tests from the memory. The controller also performs the selected one or more tests on the transmission line and detects an electrical characteristic of the transmission line.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of and claims priority from U.S.patent application Ser. No. 11/297,626, filed Dec. 7, 2005, and entitled“METHOD AND APPARATUS FOR TELEPHONE LINE TESTING,” which is incorporatedherein by reference in its entirety and which is a continuation of U.S.Pat. No. 7,003,078, which is incorporated herein by reference in itsentirety and which is a Continuation-in Part Application of U.S. patentapplication Ser. No. 09/239,591, filed on Jan. 29, 1999, now abandoned.

BACKGROUND

1. Field of the Disclosure

This disclosure relates generally to methods and devices fortransmission line testing.

2. Description of the Related Art

The characteristics of telephone lines vary greatly. Typical telephonelines connecting a customer premises to a public switch telephonenetwork (PSTN) vary in terms of length, wire gauge, amount of bridgedtap, background noise, loading coils, and other aspects. In addition,faults may be present along the telephone lines such as: a shortcircuit, an open circuit, conductor leakage, a short circuit to a powerline, or induction interference from a power line. The operation andcommunications integrity of loop transmission systems depends on thetelephone line characteristics. Loop transmission systems include aplain old telephone system (POTS), and digital subscriber line servicessuch as an integrated services digital network (ISDN), high speeddigital subscriber line (HDSL), very high speed digital subscriber line(VDSL), or asymmetric digital subscriber line (ADSL). These digitalsubscriber line services are commonly referred to as XDSL services.

Because the integrity of XDSL communications services depends on thequality of the transmission line connection, it is desirable to test thetelephone line connecting a customer premises to the PSTN to determinewhether the telephone line will support the desired transmissionservice. It is also desirable to test the line to diagnose the source oftransmission faults or interference.

Presently, two methods are commonly employed to test telephonetransmission lines: (1) central office or remote terminal automated linetest systems, and (2) a dispatched technician with a hand-held test set.In the first case, a line test command is sent from a centralized loopmaintenance system to a network terminating node (NTN) such as a localtelephone switch or carrier system located in a central office or remoteequipment site. In response, the NTN connects the line to be testedthrough a series of relays to a system that performs electricalmeasurements of the telephone transmission line. The results of thesemeasurements are then reported back to the loop maintenance system.

In the second case, a technician is dispatched to connect a hand-heldtest set to the telephone transmission line to be tested at one of thefollowing locations: (1) the central office main distributing frame, (2)the network interface device (NID) at the customer node, or (3) anintermediate point such as a serving area interface point. Using thehand-held test set, the technician measures the electricalcharacteristics of the line and reports the results of the test to theloop maintenance center. In either case, the electrical characteristicsof the line are known, and a determination can then be made as to thetype of digital communications services the telephone transmission linewill support.

There are several shortcomings, however, with the present methods forqualifying telephone transmission lines for digital communicationservices. In the first case, transmission loops served from some networkterminating nodes, such as digital subscriber line access multiplexersand digital loop carrier systems, may not provide metallic test accessto the telephone transmission line or the line measurement unit. In thecase where telephone service is not yet activated, the telephonetransmission line may not be connected to an NTN at all. In thesesituations, it would not be possible to perform an automated line testfrom the network-end of the line. Furthermore, transmission loops whichare connected to an NTN with a metallic test bus and a line measurementunit, may only respond to test frequencies within the sub-4 kHz band dueto bandwidth limitations of the test bus or the line measurement unit.In addition, background interference noise at the customer node may bedifficult to observe with testing equipment located only at the NTN.

Dispatching a technician to test the telephone transmission line has theobvious shortcoming of increasing the time and expense to providedigital communication services to customers. This results from the needfor personnel to perform these tests, and the need to providetechnicians with testing equipment.

The present disclosure overcomes the shortcomings of present telephonetransmission line testing methods by providing a modem at the customerpremises for testing and qualifying the customer connection to the PSTNfor XDSL communication services.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of one embodiment of the presentinvention used in connection with a computer located at a customerpremises.

FIG. 2 is a schematic block diagram of one embodiment of the modem foruse in the telephone line testing scenario of FIG. 1.

FIG. 3 is a perspective view of one embodiment of a direct accessarrangement testing device according to the present invention.

DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there is shown a schematic block diagram of anembodiment of the present method of testing a telephone transmissionline. The system shown in FIG. 1 includes a modem 10 located at thecustomer premises 12 which is connected by way of transmission line 14to the network interface device 16 at the customer premises 12.Transmission line 14 will typically include the modem line connected toa common telephone wall jack, and associated wiring from the wall jackto the network interface device 16. Alternatively, transmission line 14can include the modem line connected directly into the network interfacejack in the NID 16. It is contemplated that the modem 10 will typicallybe part of a digital communications device such as a computer 18 or willbe connected to such a device as shown in FIG. 1 by transmission line20. XDSL modems are commonly included in today's personal computersystems. Unlike customer-end XDSL modems to date, however, modem 10includes wideband loop testing and reporting functions. Between thenetwork interface device 16 at the customer premises 12 and the publicswitch telephone network (PSTN) 22, is the telephone transmission line24 to be tested. Of course, the PSTN could also represent a digitalnetwork.

Computer 18 is shown as part of a representative digital communicationssystem at a customer premises 12. The modem 10 is typically a necessarypart of computer 18 which allows computer 18 to transmit and receivedigital signals over telephone transmission line 24. For purposes ofline testing, however, computer 18 is not necessary if modem 10 isequipped with a user interface for displaying the results of thetelephone transmission line test. It is to be understood that computer18 is shown for illustration purposes and could be interchanged, forexample, with other equipment that generates a communications signal tobe sent over the telephone transmission line 24.

Referring to FIG. 2, an embodiment of the modem 10 comprises atransmitter/receiver 26 and direct access arrangement (DAA) 28. Thetransmitter/receiver 26 includes a modem controller 30 such as amicroprocessor, associated memory 32, application specific integratedcircuit (ASIC) 34, and a digital signal processor (DSP) 36. Thesecomponents communicate along signal paths 38, 40 and 42.

The direct access arrangement 28 includes a digital-to-analog (D/A) andanalog-to-digital (A/D) converter 44 and telephone interface circuitry46. The converter 44 communicates with the DSP 36 and interface 46 alongsignal paths 48 and 50, respectively. The interface 46 transmits signalsto and receives signals from the network interface device 16 alongtransmission line 14.

The modem controller 30, memory 32, ASIC 34, and DSP 36 define atransmitter for generating test signals on telephone transmission line24. Modem controller 30, memory 32, ASIC 34 and DSP 36 also define areceiver for detecting signals in response to test signals transmittedto telephone transmission line 24.

In addition, modem 10 preferably includes a user interface 48 incommunication with modem controller 30 along signal line 54 fordisplaying the telephone transmission line test results to a user.

In operation, customers who desire DSL services would connect the modem10 to a wall jack at the customer premises or the network interface jackin the network interface device 16. The modem 10 performs a series oftelephone line tests to qualify the line for its desired use and/or todiagnose the source of transmission interference. The test results arepresented to the user by the user interface 52 or, alternatively, can betransmitted to, for example, computer 18 for display, or alongtransmission line 24 to a communications service provider. In thismanner, the telephone transmission line 24 can be pre-qualified for thedesired communications service.

To display an output indicative of the electrical characteristics oftelephone transmission line 24, the modem 10 performs a series of tests.The testing sequence and logic is stored in memory 32 and executed bymodem controller 30 in cooperation with transmitter/receiver 26 and DAA28. The following functions are carried out by the modem 10 inqualifying the telephone transmission line 24. One function is linemonitoring which consists of measuring background noise power in one ormore frequency bands in a frequency range of approximately 0 Hz to 5MHz. Another function is measurement of AC or DC voltage between the tipand ring, tip and ground, and ring and ground terminals of the telephonetransmission line 24. Stimulus and response testing is also performed bythe modem 10 in the form of transmitting test tones, receiving responsesignals in response to the test tones, and analyzing the amplitude andphase of the signal reflections from the transmission line 24.Additionally, modem 10 transmits test pulses, receives response signalsin response to the test pulses, and analyzes the amplitude and delay ofthe pulse reflections from the transmission line 24. Additionalfunctionality includes measurement of resistance between the tip andring, tip and ground, and ring and ground terminals of transmission line24, as well as measurement of the capacitance between the tip and ringterminals of transmission line 24.

Depending upon the communication service desired by the customer, aseries of measurements could be performed with some of the testsperformed more than once, or not at all, depending on the systemconfiguration or the results of earlier tests. In addition, oralternatively, during a test sequence, the end-user could be instructedby the modem controller 30 via the user interface 48 to perform certainactions such as to place telephones on or off hook.

At the conclusion of the sequencing and analysis, a transmission linequality value is developed as a function of the test results.

One scenario for deriving the line quality value is as follows. The useris asked to indicate the type of DSL transmission system for which theline analysis is being performed. For example: HDSL, ADSL, or ISDN. Fromthis, assumptions are made for the typical transmitted frequencyband(s), signal power, modulation method, and coding, among otherthings.

The broadband attenuation of the line is estimated by applying a voltagestep to the line 24 and measuring the time-constant of the resultingcurrent flow. The time-constant estimates the line capacitance, fromwhich the line length is inferred. The estimation of the broadbandattenuation could further be refined by applying a short voltage pulseto the line and measuring the number and amplitude of the observedechoed pulses. From these pulses, the presence of bridged taps can beascertained. An additional attenuation allowance would then be made foreach bridged tap. By applying a single or multiple tone frequency sweepto the line and observing the reflected signals, nonlinear distortionand the presence of a loading coil can also be detected. In addition,the background line noise would be preferably measured in one or morefrequency bands. If the line response indicates the presence of aloading coil, then the line is not suitable for broadband DSL service.This would be indicated to the user or service-provider.

With knowledge of the nominal transmitted signal power and the estimatedline attenuation from the measurements mentioned above, the receivedsignal power is predicted. The noise power is predicted from themeasured background noise, and the measured nonlinear distortion. Apredicted signal-to-noise ratio (SNR) value is then estimated. For aknown transmission method (modulation type, transmit power, coding type,bandwidth) the achievable bit-rate is derived from the SNR. Forasymmetric transmission systems (such as ADSL), a SNR estimate isderived separately for the upstream and downstream directions. Thus, aseparate bit-rate capacity estimate is provided for each direction oftransmission.

This bit-rate capacity is then represented as a line quality value whichis then displayed to the end user by way of the user interface 48. Thecustomer could then relay the line test results to the communicationsservice provider. Alternatively, the test results could be transmittedto the service provider over transmission line 24.

With the preferred implementation of the line testing method, linetesting would be performed in a single-ended manner. In other words, thetest is conducted at the customer premises only, and no testingequipment is required at the other end of telephone transmission line24. Of course, as an alternative implementation, a double-ended testcould be performed involving coordinating testing functions at both thecustomer end of telephone transmission line 24 and the network end oftelephone transmission line 24. In the double-ended testing scenario,test signals can be transmitted and received by the modem 10 and thePSTN 22.

The testing procedures described above can be initiated by either theend user at the customer premises or by way of an initiation messagefrom the service provider or the local network provider via the DSL pathor dial-up voice band modem connection.

Referring now to FIG. 3, there is shown a perspective view of oneembodiment of a direct access arrangement device 28 according to thepresent disclosure. The device is a hand-held test set, connected by wayof a transmission line 14 to a network interface device (NID) 16 at thecustomer premises. In the example shown in FIG. 3, the transmission lineis a standard telephone line with RJ-11 connectors 60, 62 for connectingto the NID 16 and PSTN by way of the telephone transmission line 24. Ifthe device is being used at the network central office, a different typeof communication cable may be used to interface with the maindistribution frame (MDF) or switch location associated with a particularcustomers loop.

The test set is small in size and can be hand-held. For example, the setmay be 7.times.4.times.2 inches or less. For easy portability, abelt-clip 70 can be affixed to one side of the device. Preferably, thedevice is battery powered, and activated with a power switch 74 afterconnection. The user interface 100 includes two indicators such as LEDs102, 104 which preferably can each indicate red or green and can flashon and off or be lit continuously.

In operation, the test set qualifies a customer loop for XDSLcommunications, the loop being from between the ADSL terminationunit-remote (ATU-R) to the ATU-Central Office (ATU-C). Once connected,the test set performs at least several of the line tests discussedabove, including attempting to synchronize as an ADSL modem. The testset is capable of inter-operating with the Alcatel 1000 and/or Cisco6100 digital subscriber loop access multiplexers (DSLAMs), for example.

Upon power-up, LED 104 indicates that initialization is complete andpower is sufficient (solid green light), power is low (flashing greenlight), or that the set has failed its power-up initialization tests(solid or flashing red light). If power-up is successful, the test setcontinues into the testing phase. At least several of the tests outlinedabove are performed including testing for an open circuit on either thetip or ring terminal. That is, tip to ground, ring to ground and tip toring voltages are determined. During the testing phase, while the unitis performing the tests, LED 102 is blinking green to indicate that theunit is active. If all of the tests are successful and the unit hasdetermined that the customer loop qualifies for XDSL communications, LED102 is activated to be solid green. If the tests have failed, theindicator is activated as a solid or flashing red light. However, if theopen circuit test has failed, i.e., there is insufficient voltagedetected between the tip and ring circuits, the indicator alternatesflashing green and red. The alternating green/red signal thus indicatesa possible open loop on the customer circuit. If the open loop issue isresolved, the customer loop may still qualify for XDSL communicationsservices. In this way, the test unit acts as a go/no-go gauge forqualifying a customer loop either at the customer premises, or at thecentral office. When performed at the customer premises, the unit maycommunicate either the test passed, test failed, or test failed withpossible open loop results to the central office.

The hand-held test set of FIG. 3 thus provides a simple, effectivedevice for qualifying a customer loop for XDSL communication services.Of course, the user interface could take many forms, and others arecontemplated by the present invention. Preferably, however, theinterface should communicate at least whether the test has passed orfailed and whether a possible open circuit condition exists. Oneindication could accomplish this by a solid, slow blinking and fastblinking signal, respectively, for example. The test set of FIG. 3qualifies the customer loop by indicating whether the customer modemwill be able to synchronize with the network. It does not test foroptimum communications rates.

While the invention has been described in connection with one or moreembodiments, it is to be understood that the invention is not limited tothese embodiments. On the contrary, the invention covers allalternatives, modifications and equivalents as may be included withinthe scope and spirit of the appended claims.

1. A modem device comprising: a transmission line interface to connectto a transmission line; a memory to store a plurality of transmissionline tests; and a controller coupled to the memory and to thetransmission line interface, the controller to select one or more testsof the plurality of transmission line tests from the memory, thecontroller to perform the selected one or more tests on the transmissionline and to detect an electrical characteristic of the transmissionline.
 2. The modem device of claim 1, wherein the modem device is notconnected to a computer when the selected one or more tests areperformed.
 3. The modem device of claim 1, wherein the memory is acomponent of a transmitter/receiver of the modem device.
 4. The modemdevice of claim 1, further comprising: a display coupled to thecontroller, the display operable to display an output indicative of thedetected electrical characteristic.
 5. The modem device of claim 1,wherein the controller is adapted to generate a line quality value basedon the electrical characteristic.
 6. The modem device of claim 1,wherein the selected one or more tests comprise a stimulus and responsetest wherein the controller transmits test tones onto the transmissionline, receives a reflected signal from the transmission line, andanalyzes an amplitude and a phase of the reflected signal to determinethe electrical characteristic.
 7. The modem device of claim 1, whereinat least one of the plurality of transmission line tests is configuredto generate an output to a display indicative of a service qualificationbased on an achievable bit-rate estimate.
 8. The modem device of claim7, wherein the achievable bit-rate estimate is derived from an estimatedsignal-to-noise ratio value.
 9. The modem device of claim 8, wherein theestimated signal-to-noise ratio value is obtained from a predicted noisepower level and from a predicted received signal power.
 10. The modemdevice of claim 9, wherein the predicted noise power level is obtainedby the modem device by measuring background noise of the transmissionline.
 11. A method comprising: selecting a test sequence from aplurality of test sequences in a memory of a modem device based on adesired communication service; performing the test sequence on atransmission line using the modem device to determine an electricalcharacteristic of the transmission line; and generating within the modemdevice an output value indicative of the electrical characteristic. 12.The method of claim 11, wherein selecting the test sequence comprises:receiving a user instruction via a user interface of the modem device;and automatically selecting the test sequence from the plurality of testsequences in response to the user instruction.
 13. The method of claim11, wherein the test sequence is selected based at least partially on aselected communication service.
 14. The method of claim 11, whereinperforming the test sequence comprises: executing a first test operationof the selected test sequence; monitoring the transmission line todetermine a result in response to the first test operation; andexecuting a second test operation in response to the result of the firsttest operation.
 15. The method of claim 11, wherein performing the testsequence comprises: transmitting one or more stimulus signals to thetransmission line; receiving at least one reflected signal in responseto the one or more stimulus signals; and measuring a characteristic ofthe at least one reflected signal.
 16. The method of claim 15, whereinthe characteristic of the at least one reflected signal is selected fromthe group consisting of an amplitude, a phase, a delay, and a timeconstant.
 17. The method of claim 11, wherein performing the testsequence comprises: applying a voltage step to the transmission line;measuring a time constant of the transmission line to determine a linecapacitance; and inferring a length of the transmission line based onthe line capacitance.
 18. The method of claim 11, further comprisinggenerating an output to a display indicative of a service qualificationbased on an achievable bit-rate estimate.
 19. The method of claim 11,wherein performing the test sequence comprises determining an achievablebit-rate estimate derived from an estimated signal-to-noise ratio value.20. The method of claim 11, wherein performing the test sequencecomprises determining a predicted noise power level and a predictedreceived signal power.